1. Field of the Invention
The present invention relates to an amplifier, and, more particularly, to a chopping amplifier. More specifically, the present invention relates to a segmented chopping amplifier.
2. Description of Related Art
An amplifier may have several non-idealities, which affect the overall quality of signals that the amplifier processes. Some of these non-idealities are offset, 1/f noise, and thermal noise. Offset is spectrally represented as a signal with a zero frequency and an amplitude equivalent to the magnitude of the offset. The 1/f noise, as its name implies, is inversely proportional to frequency, and thermal noise is constant across all frequencies. The key feature is the frequency at which the magnitude of the 1/f noise is equal to the thermal noise. This frequency is known as the 1/f corner frequency and is typically located in the frequency domain between 1 kHz to 1 MHz for most amplifiers.
For most applications, the input signals that are applied to an amplifier are limited in frequency. For the input signals that fall below the 1/f corner frequency and have amplitudes less than the 1/f noise at the same frequency, the signal then becomes lost. Chopping techniques for amplifiers have been utilized to modulate the offset and the 1/f noise to a higher frequency (e.g., a portion of the spectrum about a chop clock frequency fchop at which no 1/f noise exist). Low pass filtering of the signal then removes the offset and the 1/f noise and ideally leaves the signal only with thermal noise. Exemplary chopping techniques have been described in “Circuit Techniques for Reducing the Effects of Op-Amp Imperfections: Autozeroing, Correlated Double Sampling, and Chopper Stabilization” by Christian C. Enz and Gabor C. Temes, IEEE Proceedings, November 1996 and U.S. Pat. No. 5,039,989 entitled “Delta-Sigma Analog-to-Digital Converter with Chopper Stabilization at the Sampling Frequency” to Welland et al.
With reference now to FIG. 1, a chopping amplifier 100 according to the prior art is shown. Chopping amplifier 100 receives a differential input signal 101 and provides a differential output signal 120. Chopping amplifier 100 has a chop clock controller 122. Chop clock controller 122 is coupled to input chopping switches 104, 106, 108, and 110 and output chopping switches 112,114, 116, and 118 and controls these switches.
Input and output chopping switches 104, 106, 108, 110, 112, 114, 116, and 118 are divided into two groups. The first group includes input chopping switches 104, 110 and output chopping switches 112, 118, which are controlled by clock signal φA of chop clock controller 122. The second group includes input chopping switches 106, 108 and output chopping switches 114, 116, which are controlled by clock signal φB of chop clock controller 122. Referring now to FIG. 2, a timing diagram for the clock signals of chop clock controller 122 according to the prior art is shown. Chop clock controller 122 generates the clock signals φA and φB according to a master chop clock signal φchop. Clock signals φA and φB are non-overlapping clock signals as shown in the timing diagram of FIG. 2. Non-overlapping clock signals φA and φB are needed to drive input and output chopping switches 104, 106, 108, 110, 112, 114, 116, and 118 and to avoid shorting of inputs and outputs due to delays. A non-overlap period 202 illustrates the nature of non-overlapping clock signals φA and φB.
Chopping amplifier 100 modulates input signal 101 to a higher portion of the frequency spectrum, such as a chop clock frequency fchop of chop clock signal φchop. Generally, no 1/f noise exists at the chop clock frequency fchop. Chopping amplifier 100 amplifies input signal 101 and adds the 1/f noise and the thermal noise to produce an output signal 121 (before output switches 112, 114, 116, and 118). Output signal 121 of chopping amplifier 100 is modulated by the output chopping switches 112, 114, 116, and 118. The net effect of the switching by output chopping switches 112, 114, 116, and 118 is the demodulation of the input signal back to the baseband (e.g., f=0) and the modulation of the 1/f noise and the offset to the higher frequency fchop where they are removed by low pass filtering. Thus, chopping amplifier 100 ideally eliminates errors due to the 1/f noise and offset during the amplification process.
However, chopping amplifier 100 has some non-idealities that could lead to distortion, excess noise above and beyond the thermal noise, and/or residual offset. For example, non-idealities exist in the asymmetries between clock signals φA and φB and when chopping amplifier 100 is operating in the open loop during the non-overlap periods (e.g., non-overlap period 202).
In a traditional chopping scheme, operational amplifier 102 is operating in an open loop (e.g., all input and output chopping switches 104, 106, 108, 110, 112, 114, 116, and 118 are open) during the non-overlap periods. This open loop situation can cause the output of operational amplifier 102 to runaway. Depending upon the nature of the runaway, distortion, noise, and/or residual offset may result. Furthermore, during the non-overlap period, input signal 101 is sampled and held at the input of operational amplifier 102. Any broadband noise near the edge rate (e.g., twice the chopping frequency 2*fchop of chopping clock signal φchop) gets aliased down to the frequency baseband, which causes severe degradation of dynamic range and linearity. This aliasing of the noise is especially problematic in over-sampled data converters which have a large amount of shaped quantization noise at or near twice the chopping frequency 2*fchop.
The present invention recognizes the desire and need for providing an improved chopping amplifier. The present invention further recognizes the desire and need to provide a chopping amplifier that resolves the open loop problem and avoids the runaway situation. The present invention also recognizes the need and desire for a chopping amplifier that reduces aliasing of noise to the frequency baseband and the magnitude of chopping artifacts. The present invention overcomes the problems and disadvantages in accordance with the prior art.